A PRESERVED MESOPROTEROZOIC OXYCLINE AND A HOSPITABLE LACUSTRINE ENVIRONMENT FOR EARLY EUKARYOTES

Terrestrial environments have been suggested as an oxic haven for eukaryotic life and diversification during the “boring billion” years of the Proterozoic Eon when oceanic environments were dominantly anoxic. However, recent iron speciation data from the ~1.1 Ga Nonesuch Formation, deposited in a large lake in the interior of North America, are interpreted to indicate an anoxic water column, similar to ocean environments at the time with only the surficial-most waters being oxygenated (Cummings et al. 2013). In order to shed new light on these distinct hypotheses, we analyzed two drill-cores through the Nonesuch Formation in Wisconsin from a well-constrained stratigraphic framework spanning the transgression into lacustrine sediments and then shallowing into the overlying fluvial sediments (Stewart and Mauk, 2017). This lake is known to have been home to eukaryotes based on a diverse fossil assemblage (Wellman and Strother, 2015) and is well-preserved with a maximum burial temperature of < 155°C (Gallagher et al., 2017). Rock magnetic analyses highlight three distinct facies that provide a record of the oxycline in the lake. In the deepest waters, reductive dissolution of iron oxides, likely due to dissimilatory iron reducing bacteria, records an anoxic environment either in the water column or restricted to sediment pore water. In the shallowest waters of the lake and in overlying fluvial lithologies, every iron oxide has been oxidized to its most oxidized form, hematite. At intermediate water depths in the lake, a mixed-phase facies with hematite and the mixed-valence oxide, magnetite, indicates widespread dysoxic to suboxic conditions rather than anoxia. By pairing rock magnetic analyses with petrography, paleomagnetism, and iron speciation, we can delve into the mineralogy to understand authigenic versus detrital formation mechanisms and can compare these results to prior analyses of samples from Michigan. Our data suggest that for much of its temporal duration and throughout much of its water column, Lake Nonesuch was hospitable to early eukaryotes, providing fresh insight into iron cycling and redox conditions in terrestrial environments during mid-Proterozoic time.